U.S. Pat. Nos. 3,822,238 and 3,975,350 describe an active agent carrier system comprising an active agent and as a carrier vehicle therefor, a hydrophilic, preferably cross-linked polyurethane polymer. The carrier system is utilized by subjecting it to aqueous conditions whereby the polyurethane carrier undergoes hydration and swelling with concomitant formation of an insoluble hydrogel from which the active agent is leached out by and into the aqueous medium. The rate and duration of release can be controlled by employing a carrier polymer of the requisite water absorptivity. Generally speaking, polyurethane polymers prepared from water soluble active hydrogen resins, as exemplified by polyoxyethylene polyols, will exhibit the highest water absorptivity. Polymers having diminished water absorptivity can be produced from less soluble resins, or by crosslinking.
The carrier system aforesaid is useful as a means for delivering various active agents, such as medicinal or cosmetic agents, to a treatment zone. For instance, shaped articles made of the carrier system can be inserted into the body to provide precision administering of drugs or serums over extended periods of time.
Another class of hydrophilic polyurethanes that form insoluble hydrogels in the hydrated state is disclosed in U.S. Pat. Nos. 4,156,066 and 4,156,067. These polyurethanes are characerized by the presence of lactone groups in the polymer backbone. The lactone may be opened by hydrolytic cleavage to form carboxylic acid groups which render the polymer soluble in alkaline medium. Other specialized hydrophilic polyurethanes are the polyurethane diacrylates of U.S. Pat. No. 4,359,558 and the polyurethane quaternary ammonium salts of U.S. Pat. No. 4,451,635.